Instructions
Flip the button over and examine the back for markings from the manufacturer.
Look for a logo that says ̶0;Scovill MF̵7;G CO. *WaterBury.* This is usually an indicator of a button that was made between 1902 and 1918.
Flip the button back over to examine the front.
Look at the details on the eagle. While earlier buttons featured a long-necked eagle craning its head above his wings, the eagles on 1902-era buttons have a much shorter neck and head that fall well below their wings.